1. Field of the Invention
This invention relates to a rate converter for converting a rate of data in order to carry out transmission/reception (transfer) of data between digital circuits operative at two different clock rates.
2. Description of the Related Art
Generally, in order to carry out transmission and reception of data between digital circuits operative at two different clock rates, a rate converter for converting a rate of data is required.
For example, in the case of carrying out transmission and reception of digital video signals between a digital video signal processing circuit of an imaging apparatus operative at a clock rate of 18 MHz and a digital video signal processing circuit of a digital video tape recorder in conformity with the D1 standard operative at a clock rate of 13.5 MHz, rate converters such as a down-rate converter for converting a rate of a digital video signal outputted from the imaging apparatus from 18 MHz to 13.5 MHz, and an up-rate converter for converting a rate of a digital video signal outputted from the digital video tape recorder from 13.5 MHz to 18 MHz, etc. are required.
A conventional rate converter is adapted to apply up-conversion to input data so that its clock rate becomes equal to a clock rate of the least common multiple of an input clock rate and an output clock rate to implement thinning processing thereto by using a filter to thereby provide output data of a target output clock rate. For such a conventional rate converter, a filtering processing at the clock rate of the least common multiple is required.
For example, in a down-rate converter, input data having a clock rate of 18 MHz is converted to output data having a clock rate of 13.5 MHz by a filtering processing as shown in FIGS. 1 and 2.
Namely, in the down-rate converter, 0 data are first inserted into portions which can be sampling points of 13.5 MHz, as shown at B in FIG. 1, with respect to input data {X.sub.n } having a clock rate of 18 MHz as shown at A in FIG. 1 to up-convert the clock rate of the input data {X.sub.n } so as to become equal to a frequency of the least common multiple of 18 MHz and 13.5 MHz, i.e., a clock rate of 54 MHz. Thus, in the frequency region, frequency components which have repeated with a frequency of 18 MHz being as a unit as shown at A of FIG. 2 change into frequency components repeating with a frequency of 54 MHz being as a unit while the frequency characteristic is as it is as shown at B of FIG. 2.
Then, a filter of a characteristic as shown at C of FIG. 1 and C of FIG. 2 is applied to the data having the clock rate of 54 MHz. Namely, since the output clock rate is 13.5 MHz, if there is any frequency component more than 6.75 MHz which is one half of 13.5 MHz in a frequency range up to 27 MHz which is one half of 54 MHz, any arising noise or error might take place by the sampling theorem when the clock rate is caused to be 13.5 MHz, thus failing to maintain the original frequency characteristic. For this reason, a low-pass filter for suppressing frequency components more than 6.75 MHz is applied.
Here, data {Y.sub.n } having a clock rate of 54 MHz which has suppressed frequency components more than 6.75 MHz is subjected to a filtering processing of a transfer function F.sub.1 (z) indicated by the following formula (1) where, e.g., the number of taps is set to 12 in a digital filter operative at 54 MHz with respect to input data X.sub.n =Z.sup.n.X.sub.1 ##EQU1##
By implementing the filtering processing of the transfer function F.sub.1 (z) indicated by the above-mentioned formula (1) to data {Y.sub.n }, data Y.sub.1 .about.Y.sub.14 as indicated by the following formula (2) are provided. EQU Y.sub.1 =k.sub.2 .multidot.X.sub.4 +k.sub.5 .multidot.X.sub.3 +k.sub.8 .multidot.X.sub.2 +k.sub.11 .multidot.X.sub.1 EQU Y.sub.2 =k.sub.0 .multidot.X.sub.5 +k.sub.3 +X.sub.4 +k.sub.6 .multidot.X.sub.3 +k.sub.9 .multidot.X.sub.2 EQU Y.sub.3 =k.sub.1 .multidot.X.sub.5 +k.sub.4 .multidot.X.sub.4 +k.sub.7 .multidot.X.sub.3 +k.sub.10 .multidot.X.sub.2 EQU Y.sub.4 =k.sub.2 .multidot.X.sub.5 +k.sub.5 .multidot.X.sub.4 +k.sub.8 .multidot.X.sub.3 +k.sub.11 .multidot.X.sub.2 EQU Y.sub.5 =k.sub.0 .multidot.X.sub.6 +k.sub.3 .multidot.X.sub.5 +k.sub.6 .multidot.X.sub.4 +k.sub.9 .multidot.X.sub.3 EQU Y.sub.6 =k.sub.1 .multidot.X.sub.6 +k.sub.4 .multidot.X.sub.5 +k.sub.7 .multidot.X.sub.4 +k.sub.10 .multidot.X.sub.3 EQU Y.sub.7 =k.sub.2 .multidot.X.sub.6 +k.sub.5 .multidot.X.sub.5 +k.sub.8 .multidot.X.sub.4 +k.sub.11 .multidot.X.sub.3 ( 2) EQU Y.sub.8 =k.sub.0 .multidot.X.sub.7 +k.sub.3 .multidot.X.sub.6 +k.sub.6 .multidot.X.sub.5 +k.sub.9 .multidot.X.sub.6 EQU Y.sub.9 =k.sub.1 .multidot.X.sub.7 +k.sub.4 .multidot.X.sub.6 +k.sub.7 .multidot.X.sub.5 +k.sub.10 .multidot.X.sub.5 EQU Y.sub.10 =k.sub.2 .multidot.X.sub.7 +k.sub.5 .multidot.X.sub.6 +k.sub.8 .multidot.X.sub.5 +k.sub.11 .multidot.X.sub.6 EQU Y.sub.11 =k.sub.0 .multidot.X.sub.8 +k.sub.3 .multidot.X.sub.7 +k.sub.6 .multidot.X.sub.6 +k.sub.9 .multidot.X.sub.5 EQU Y.sub.12 =k.sub.1 .multidot.X.sub.8 +k.sub.4 .multidot.X.sub.7 +k.sub.7 .multidot.X.sub.6 +k.sub.10 .multidot.X.sub.6 EQU Y.sub.13 =k.sub.2 .multidot.X.sub.8 +k.sub.5 .multidot.X.sub.7 +k.sub.8 .multidot.X.sub.6 +k.sub.11 .multidot.X.sub.5 EQU Y.sub.14 =k.sub.0 .multidot.X.sub.9 +k.sub.3 .multidot.X.sub.8 +k.sub.6 .multidot.X.sub.7 +k.sub.9 .multidot.X.sub.6
By taking out data at a clock rate of 13.5 MHz as shown at E of FIG. 1 from data {Y.sub.n } of a clock rate of 54 MHz as shown at D of FIG. 1 and D of FIG. 2 thus obtained, output data having a clock rate of 13.5 MHz which has maintained the frequency characteristic of input data {X.sub.n } at its maximum can be obtained as shown at E of FIG. 2.
On the other hand, in the up-rate converter, input data having a clock rate of 13.5 MHz is converted to output data having a clock rate of 18 MHz by a filtering processing as shown in FIGS. 3 and 4.
Namely, also in the up-rate converter, 0 data are inserted into the portions which can be sampling points of 18 MHz as shown at B of FIG. 3 with respect to input data {X.sub.n } of a clock rate of 13.5 MHz as shown at A of FIG. 3 to up-convert the clock rate of the input data {X.sub.n } so as to become equal to a frequency of the least common multiple of 13.5 MHz and 18 MHz, i.e., a clock rate of 54 MHz. Thus, in the frequency region, frequency components which have repeated with a frequency of 13.5 MHz being as a unit as indicated at A of FIG. 4 change into frequency components repeating with a frequency of 54 MHz being as a unit while the frequency characteristic is as it is as shown at B of FIG. 4.
A filter of a characteristic as shown at C of FIG. 3 and C of FIG. 4 is applied to the data of the clock rate of 54 MHz. Namely, since the output clock rate is 18 MHz, if there is any frequency component more than 9 MHz which is one half of 18 MHz in the frequency range up to 27 MHz which is one half of 54 MHz, any arising frequency component might take place by the sampling theorem when the clock rate is caused to be a clock rate of 18 MHz, failing to maintain the original frequency characteristic. For this reason, a low-pass filter for suppressing frequency components more than 9 MHz is applied.
Here, data {Y.sub.n } having a clock rate of 54 MHz which has suppressed the frequency components more than 9 MHz is subjected to a filtering processing of a transfer function F.sub.2 (z) indicated by the following formula (3) where, e.g., the number of taps is set to 12 in a digital filter operative at 54 MHz with respect to input data X.sub.n =z.sub.n .multidot.X.sub.1. ##EQU2##
By implementing the filtering processing of the transfer function F.sub.2 (z) as indicated by the above-mentioned formula (3) to data {Y.sub.n }, data Y.sub.1 .about.Y.sub.14 as indicated by the following formula (4) are provided. EQU Y.sub.1 =k.sub.3 .multidot.X.sub.3 +k.sub.7 .multidot.X.sub.2 +k.sub.11 .multidot.X.sub.1 EQU Y.sub.2 =k.sub.0 .multidot.X.sub.4 +k.sub.4 .multidot.X.sub.3 +k.sub.8 .multidot.X.sub.2 EQU Y.sub.3 =k.sub.1 .multidot.X.sub.4 +k.sub.5 .multidot.X.sub.3 +k.sub.9 .multidot.X.sub.2 EQU Y.sub.4 =k.sub.2 .multidot.X.sub.4 +k.sub.6 .multidot.X.sub.3 +k.sub.10 .multidot.X.sub.2 EQU Y.sub.5 =k.sub.3 .multidot.X.sub.4 +k.sub.7 .multidot.X.sub.3 +k.sub.11 .multidot.X.sub.2 EQU Y.sub.6 =k.sub.0 .multidot.X.sub.5 +k.sub.4 .multidot.X.sub.4 +k.sub.8 .multidot.X.sub.3 EQU Y.sub.7 =k.sub.1 .multidot.X.sub.5 +k.sub.5 .multidot.X.sub.4 +k.sub.9 .multidot.X.sub.3 ( 4) EQU Y.sub.8 =k.sub.2 .multidot.X.sub.5 +k.sub.6 .multidot.X.sub.4 +k.sub.10 .multidot.X.sub.3 EQU Y.sub.9 =k.sub.3 .multidot.X.sub.5 +k.sub.7 .multidot.X.sub.4 +k.sub.11 .multidot.X.sub.3 EQU Y.sub.10 =k.sub.0 .multidot.X.sub.6 +k.sub.4 .multidot.X.sub.5 +k.sub.8 .multidot.X.sub.4 EQU Y.sub.11 =k.sub.1 .multidot.X.sub.6 +k.sub.5 .multidot.X.sub.5 +k.sub.9 .multidot.X.sub.4 EQU Y.sub.12 =k.sub.2 .multidot.X.sub.6 +k.sub.6 .multidot.X.sub.5 +k.sub.10 .multidot.X.sub.4 EQU Y.sub.13 =k.sub.3 .multidot.X.sub.6 +k.sub.7 .multidot.X.sub.5 +k.sub.11 .multidot.X.sub.4 EQU Y.sub.14 =k.sub.0 .multidot.X.sub.7 +k.sub.4 .multidot.X.sub.6 +k.sub.8 .multidot.X.sub.5
By taking out data at a clock rate of 18 MHz as shown at E of FIG. 3 from the data {Y.sub.n } of the clock rate of 54 MHz as shown at D of FIG. 3 and D of FIG. 4 thus obtained, output data of a clock rate of 18 MHz which has maintained the frequency characteristic of input data {X.sub.n } at its maximum can be provided as shown at E of FIG. 4.
Meanwhile, as described above, the conventional rate converter requires an operational processing unit operative at a high speed for implementing a filtering processing at a clock rate of the least common multiple of an input clock and an output clock to input data.
Here, in a down-rate converter for converting input data of a clock rate of 18 MHz to output data .of a clock rate of 13.5 MHz, data {Y.sub.n } of a clock rate of 54 MHz which is the least common multiple of the input clock rate of 13.5 MHz and the output clock rate of 18 MHz obtained by the filtering processing of the transfer function F.sub.1 (z) indicated by the above-described formula (1) can be classified into three sets every coefficient.
The first set is comprised of data {Y.sub.3n-1 } having coefficients {k.sub.0, k.sub.3, k.sub.6, k.sub.9 } indicated by the following formula (5): EQU Y.sub.2 =k.sub.0 .multidot.X.sub.5 +k.sub.3 .multidot.X.sub.4 +k.sub.6 .multidot.X.sub.3 +k.sub.9 .multidot.X.sub.2 EQU Y.sub.5 =k.sub.0 .multidot.X.sub.6 +k.sub.3 .multidot.X.sub.5 +k.sub.6 .multidot.X.sub.4 +k.sub.9 .multidot.X.sub.3 EQU Y.sub.8 =k.sub.0 .multidot.X.sub.7 +k.sub.3 .multidot.X.sub.6 +k.sub.6 .multidot.X.sub.5 +k.sub.9 .multidot.X.sub.6 ( 5) EQU Y.sub.11 32 k.sub.0 .multidot.X.sub.8 +k.sub.3 .multidot.X.sub.7 +k.sub.6 .multidot.X.sub.6 +k.sub.9 .multidot.X.sub.5 EQU Y.sub.14 =k.sub.0 .multidot.X.sub.9 +k.sub.3 .multidot.X.sub.8 +k.sub.6 .multidot.X.sub.7 +k.sub.9 .multidot.X.sub.6
The second set is comprised of data {Y.sub.3n } having coefficients {k.sub.1, k.sub.4, k.sub.7, k.sub.10 } indicated by the following formula (6): EQU Y.sub.3 =k.sub.1 .multidot.X.sub.5 +k.sub.4 .multidot.X.sub.4 +k.sub.7 .multidot.X.sub.3 +k.sub.10 .multidot.X.sub.2 EQU Y.sub.6 =k.sub.1 .multidot.X.sub.6 +k.sub.4 .multidot.X.sub.5 +k.sub.7 .multidot.X.sub.4 +k.sub.10 .multidot.X.sub.3 EQU Y.sub.9 =k.sub.1 .multidot.X.sub.7 +k.sub.4 .multidot.X.sub.6 +k.sub.7 .multidot.X.sub.5 +k.sub.10 .multidot.X.sub.5 ( 6) EQU Y.sub.12 =k.sub.1 .multidot.X.sub.8 +k.sub.4 .multidot.X.sub.7 +k.sub.7 .multidot.X.sub.6 +k.sub.10 .multidot.X.sub.6
The third set is comprised of data {Y.sub.3n-2 } having coefficients {k.sub.2, k.sub.5, k.sub.8, k.sub.11) indicated by the following formula (7): EQU Y.sub.1 =k.sub.2 .multidot.X.sub.4 +k.sub.5 .multidot.X.sub.3 +k.sub.8 .multidot.X.sub.2 +k.sub.11 .multidot.X.sub.1 EQU Y.sub.4 =k.sub.2 .multidot.X.sub.5 +k.sub.5 .multidot.X.sub.4 +k.sub.8 .multidot.X.sub.3 +k.sub.11 .multidot.X.sub.2 EQU Y.sub.7 =k.sub.2 .multidot.X.sub.6 +k.sub.5 .multidot.X.sub.5 +k.sub.8 .multidot.X.sub.4 +k.sub.11 .multidot.X.sub.3 ( 7) EQU Y.sub.10 =k.sub.2 .multidot.X.sub.7 +k.sub.5 .multidot.X.sub.6 +k.sub.8 .multidot.X.sub.5 +k.sub.11 .multidot.X.sub.6 EQU Y.sub.13 =k.sub.2 .multidot.X.sub.8 +k.sub.5 .multidot.X.sub.7 +k.sub.8 .multidot.X.sub.6 +k.sub.11 .multidot.X.sub.5
The data {Y.sub.3n-1 =56 having the first set of coefficients {k.sub.0, k.sub.3, k.sub.6, k.sub.9 } can be obtained by a digital filter of a transfer function Fa(z) indicated by the following formula (8): EQU Fa(z)=k.sub.0 +k.sub.3 .multidot.z.sup.-1 +k.sub.6 .multidot.z.sup.-2 +k.sub.9 .multidot.z.sup.-3 ( 8)
Further, the data {Y.sub.3n } having the second set of coefficients {k.sub.1, k.sub.4, k.sub.7, k.sub.10 } can be obtained by a digital filter of a transfer function Fb(z) indicated by the following formula (9): EQU Fb(z)=k.sub.1 +k.sub.4 .multidot.z.sup.-1 +k.sub.7 .multidot.z.sup.-2 +k.sub.10 .multidot.z.sup.-3 ( 9)
In addition, the data {Y.sub.3n-2 } having the third set of coefficients {k.sub.2, k.sub.5, k.sub.8, k.sub.11 } can be obtained by a digital filter of a transfer function Fc(z) indicated by the following formula (10): EQU Fc(Z)=k.sub.2 +k.sub.5 .multidot.z.sup.-1 +k.sub.8 .multidot.z.sup.-2 +k.sub.11 .multidot.z.sup.-3 ( 10)
Accordingly, in the down-rate converter, in place of inserting 0 data with respect to input data of a clock rate of 18 MHz to up-convert the clock rate so as to become equal to 54 MHz which is the least common multiple, three digital filters for carrying out filtering processing of respective transfer functions Fa(z), Fb(z) and Fc(z) indicated by the above-mentioned formulas (8), (9) and (10) are caused to be operative in parallel at 18 MHz of the input clock rate, thereby making it possible to calculate the data {Y.sub.n }.
Similarly, in an up-rate converter for converting input data of a clock rate of 13.5 MHz to output data of a clock rate of 18 MHz, data {Y.sub.n } of a clock rate of 54 MHz which is the least common multiple of the input clock rate of 13.5 MHz and the output clock rate of 18 MHz obtained by the filtering processing of the transfer function F.sub.2 (z) indicated by the above-mentioned formula (3) can be classified into four sets of data {Y.sub.4n-2 } having the first set of coefficients {K.sub.0, K.sub.4, K.sub.8 }, data {Y.sub.4n-1 } having the second set of coefficients {K.sub.1, K.sub.5, K.sub.9 }, data {Y.sub.4n } having the third set of coefficients {K.sub.2, K.sub.6, K.sub.10 }, and data {Y.sub.4n-3 } having the fourth set of coefficients {K.sub.3, K.sub.7, K.sub.11 }. In place of inserting 0 data with respect to input data of the clock rate of 13.5 MHz to up-convert the clock rate so as to become equal to 54 MHz which is the least common multiple, four digital filters for carrying out filtering processing of respective transfer functions Fa(z), Fb(z), Fc(z), Fd(z) indicated by the following formulas (11), (12), (13), (14) are caused to be operative in parallel at 13.5 MHz of the input clock rate, thereby making it possible to calculate the data {Y.sub.n }. EQU Fa(z)=k.sub.0 +k.sub.4 .multidot.z.sup.-1 +k.sub.8 .multidot.z.sup.-2( 11) EQU Fb(z)=k.sub.1 +k.sub.5 .multidot.z.sup.-1 +k.sub.8 .multidot.z.sup.-2( 12) EQU Fc(z)=k.sub.2 +k.sub.6 .multidot.z.sup.-1 +k.sub.10 .multidot.z.sup.-2( 13) EQU Fd(z)=k.sub.3 +k.sub.7 .multidot.z.sup.-1 +k.sub.11 .multidot.z.sup.-2( 14)
As described above, a plurality of digital filters are caused to be operative in parallel at an input clock rate, thereby allowing a digital filter operative at a high speed at a clock rate of the least common multiple of the input clock rate and the output clock rate to become unnecessary. However, such a plurality of digital filters are required.